Optical properties of the “clearest” natural waters

Abstract

Optical measurements within both the visible and near ultraviolet (UV) parts of the spectrum (305-750 nm) were recently made in hyperoligotrophic waters in the South Pacific gyre (near Easter Island). The diffuse attenuation coefficients for downward irradiance, K d (l), and the irradiance reflectances, R(l), as derived from hyperspectral (downward and upward) irradiance measurements, exhibit very uncommon values that reflect the exceptional clarity of this huge water body. The K d (l) values observed in the UV domain are even below the absorption coefficients found in current literature for pure water. The R(l) values (beneath the surface) exhibit a maximum as high as 13% around 390 nm. From these apparent optical properties, the absorption and backscattering coefficients can be inferred by inversion and compared to those of (optically) pure seawater. The total absorption coefficient (a tot ) exhibits a flat minimum (, 0.007 m 21 ) around 410-420 nm, about twice that of pure water. At 310 nm, a tot may be as low as 0.045 m 21 , i.e., half the value generally accepted for pure water. The particulate absorption is low compared to those of yellow substance and water and represents only ,15% of a tot in the 305-420-nm domain. The backscattering coefficient is totally dominated by that of water molecules in the UV domain. Because direct laboratory determinations of pure water absorption in the UV domain are still scarce and contradictory, we determine a tentative upper bound limit for this elusive coefficient as it results from in situ measurements.

Parameterization of the light absorption properties of chromophoric dissolved organic matter in the Baltic Sea and Pomeranian lakes

“…(13) (Ferrari and Tassan, 1992;Tassan 1994;Vodacek et al, 1997;Morel et al, 2007;Morel and Gentili, 2009;Bricaud et al, 2010;Organelli et al, 2014). We compared the a CDOM (λ)-Chl a relationship that we derived with some of the relationships between a CDOM (λ) and Chl a derived by various authors for different water types.…”

“…The relationships derived by other authors were found unsuitable for estimating CDOM absorption in the Baltic Sea and lake waters. The empirical relationships derived by Tassan (1994), Morel et al (2007), Morel and Gentili (2009) and Bricaud et al (2010) all underestimated CDOM absorption in the Baltic Sea. Such a great discrepancy between estimated and observed CDOM absorption values has resulted from the fact that these relationships were developed for clear oceanic waters, where the contribution of dissolved organic material to the total light absorption was less than in the Baltic Sea and the concentration of Chl a did not exceed 40 mg m −3 .…”

“…9). In all cases, these relationships were approximated by power functions and assumed different rates of increase in a CDOM (λ) with increasing Chl a (Tassan, 1994;Morel et al, 2007;Morel and Gentili, 2009;Bricaud et al, 2010). The relationships derived by other authors were found unsuitable for estimating CDOM absorption in the Baltic Sea and lake waters.…”

“…The effect of CDOM absorption is mostly visible in the UV and blue spectral range of electromagnetic radiation, where the CDOM contribution to the total non-water absorption can be as much as 90 %, even in the clearest natural waters found in the South Pacific subtropical gyre south of Easter Island (Morel et al, 2007;Bricaud et al, 2010;Tedetti et al, 2010). The CDOM absorption band also overlaps the primary phytoplankton pigment absorption band in the blue part of the spectrum; this leads to significant errors in standard algorithms for retrievals of chlorophyll a, especially in coastal ocean and shelf waters and semi-enclosed seas (Darecki and Stramski, 2004;Siegel et al, 2005).…”

On the influence of wind and waves on underwater irradiance fluctuations

“…The presented simulations are carried out for perpendicular solar irradiation and for an extreme clear atmosphere, where the diffuse skylight due to atmospheric Rayleigh and Mie scattering accounts for 10 % only (the sky illumination is assumed to be isotropic). The water body under consideration is very clear and oligotrophic; the (wavelengthdepending) inherent optical properties of the well-mixed seawater are derived from a chlorophyll a concentration Chl of 0.1 mg m −3 (Morel et al, 2007;Morel, 2009). The resulting diffuse attenuation coefficient for the downward irradiance K d (490 nm) is approximately 0.038 m −1 , relating to a flat sea surface.…”